1. Field of the Invention
This invention relates to a drive controlling apparatus and more particularly to a drive controlling apparatus for stopping accurately in a target position a driven body driven by a motor.
2Related Background Art
A drive controlling apparatus for controlling the movement of a driven body driven by a motor has been already extensively used for electronic devices in various fields and is used mostly for an automatic focus adjusting apparatus for controlling a taking lens to move to a focusing target position, for example, in a camera. In this kind of automatic focus adjusting apparatus, by the technical means suggested in Japanese Patent Application Laid Open No. 153526/1988 filed by the present applicant, there is adopted a limited accelerating method whereby a motor drive is controlled by comparing the value of the curve of a deceleration curve characteristic and the value of the moving speed of an actual taking lens with each other and further, at the time point when the taking lens comes near to the focusing position, the on-time of the motor is limited to a fixed time to prevent the excess movement by the inertia when the motor stops.
However, as the above-mentioned limited acceleration is made for a fixed time irrespective of the moving speed and load of the taking lens, the acceleration will be excessive or short. While the motor is switched on, the program will be only counting the time, therefore the signal of the detecting means detecting the above-mentioned moving speed will not be able to be seen and the detection of the moving speed has been likely to be wrong.
Therefore, by the technical means suggested in U.S. Pat. No. 4,878,077 by the present applicant (i.e. assignee), the on-time of the motor is varied in response to the moving speed of the taking lens and the speed can be simultaneously positively detected. That is to say, the lens moving speed by the motor is detected by the speed detecting means, this detected moving speed and the curve of the deceleration characteristic memorized by an optimum memorizing means to stop the taking lens in the target position are compared with each other and the taking lens is moved by the lens controlling means to the focusing position along the curve of the above-mentioned deceleration characteristic. In case the moving speed of the taking lens is higher than the memorized deceleration curve, the motor will be braked but, in case the moving speed is lower, depending on the degree, the motor will be switched on or off so that the speed may be controlled along the deceleration curve. Near the target within the range of the above mentioned deceleration curve, when the motor is on, in order to prevent the acceleration by continuously switching on the motor and to obtain an optimum acceleration in the moving speed at this time point, a motor switching on controlling means will be operated by the signal of the above-mentioned speed detecting means to forcibly switch off (or brake) the motor.
Now, in the technical means suggested in the above-mentioned Japanese Patent Application Laid Open No. 153526/1988 and U.S. Pat No. 4,878,077, a photointerrupter is used as a detecting element for detecting the displacement of the taking lens. That is to say, many slits are provided at regular intervals in the peripheral direction on the peripheral edge of a disc member rotating with the movement of the taking lens and are counted by a photointerrupter (abbreviated as PI hereinafter) made by arranging a light emitting part and light receiving part as opposed to each other with the moving path of these slits between them and the lens displacement is detected by the obtained output signal of the photointerrupter. The moving speed of the taking lens detected by measuring the pulse intervals of the above-mentioned PI output signal.
Now, in order to improve the detecting precision of the PI mechanism detecting the displacement of the above-mentioned taking lens, it is necessary to increase the PI output signal for the unit movement of the taking lens. Therefor it is considered (1) to elevate the driving gear ratio of the rotating slit disc or (2) to reduce the thickness of the slits of the disc and increase the number of the slits. However, by such technical means, the frequency of the PI signal will become high, the excessive responding characteristic of the waveform shaping circuit will not be followed and therefore there will be produced (I) a problem of the slit working technique and (II) a limit to the processing speed of the CPU.
If the width of the slit is left as it is but the diameter of the disc member is made large, the slit working technique among them will be solved but then the contour dimension and hence the disc will become so large that the portability of the camera will be reduced. If the clock frequency of the CPU is elevated, the processing speed of the CPU will be solved. However, if it is considered that, when the motor is driven, the power source voltage will occasionally drop, there will be reasons that, in order that the CPU may positively carry out its process even under a low voltage, it will be necessary to reduce the clock frequency and therefore the operating speed can not be generally elevated.
On the other hand, in such taking lens displacement detecting means as is described above by the PI mechanism, there is another problem that the PI pulse width is too different between the high speed driving time far from the target position and the low speed driving time near the target position. That is to say, if the high speed driving time is considered heavy, the detecting precision at the low speed driving time will become low but, if the low speed driving time is considered heavy, the clock will not be able to be detected at the high speed driving time and a physical limit will be produced.